The general objective is to understand the detailed processes involved in cellular repair of DNA and their effects on the radiation response of cells. This includes the photoenzymatic repair of ultraviolet damage (the simplest mechanism and the most amenable to analysis), and their interaction with the excision-resynthesis and recombination type repairs. The methodology utilizes the light-dependence of photoenzymatic repair to unravel individual steps in the enzyme reaction: viz., the light-independent attachment step as models of other nucleic acid-enzyme (including repair enzyme) attachments, and the light-dependent step as an example of nucleic acid photochemistry. The project deals specifically with cloning the gene for photoreactivating enzyme of E. coli, with repair and UV-sensitive mutants in mammalian cells, and with repair processes in bacterial spores.